1. Field of the Invention
The present invention relates to synchronous data transmission whereby a digital bit stream is transmitted at a high data rate in a code with parity checking, that is to say maintaining constant the parity of a specified type of the state of data of the digital bit stream transmitted by means of parity words appearing in the digital bit stream at specific times separated by a constant or variable duration. It is more particularly concerned with the transmission of an engineering service channel low data rate digital bit stream through the intermediary of the channel constituted by the parity words.
2. Description of the Prior Art
Codes with parity checking introduce into the line signal a parity law violations of which, in the event of line errors, lend themselves to analog detection as described, for example, in U.S. Pat. No. 4,121,195 and entailing the detection of variations that they cause in the longer term in the DC component of the output signal of a bistable circuit changing state on each of said specified type states of the data in the digital bit stream transmitted.
It is already known to transmit a low data rate digital bit stream for a service channel through the intermediary of the parity channel of a digital bit stream transmitted in a code with parity checking either as described in U.S. Pat. No. 4,387,460 by modulating a transmitted carrier by regularly spaced isolated parity word modifications leading to a periodic violation of the parity law at a frequency that is low in comparison with the frequency of occurrence of the parity words or as described in U.S. Pat. No. 4,633,464 by using the locations of regularly spaced isolated parity words to transmit oversampled data of the service channel low data rate digital bit stream. Both methods have the disadvantage of disrupting analog detection of line errors as this is highly sensitive to isolated violations of the parity law.
An object of the present invention is to avoid this disadvantage and to enable analog detection of line errors during the transmission of a service channel low data rate digital bit stream through the intermediary of the parity channel of a high data rate digital bit stream transmitted in a code with parity checking.